The deforestation that has stripped the Amazon since the 1970s has also exposed a long-hidden secret lurking underneath thick rain forest: flawlessly designed geometric shapes spanning hundreds of yards in diameter.

For some scholars of human history in Amazonia, the geoglyphs in the Brazilian state of Acre and other archaeological sites suggest that the forests of the western Amazon, previously considered uninhabitable for sophisticated societies partly because of the quality of their soils, may not have been as “Edenic” as some environmentalists contend.

Still, the geoglyphs, located at a crossroads between Andean and Amazonian cultures, remain an enigma.